The overall objective of this thesis was to understand better the mechanisms that shape the diversity in colour vision of fish, and to explore the adaptive significance of this divergence. Among the vertebrates, teleost fish show the greatest diversity in colour vision systems. The cichlid model system illustrates that the visual system of fish may differ among species, sexes, individuals, and life stages of individuals. The large number of available cone opsin genes, which have resulted from multiple opsin gene duplications, facilitates this high degree of variation in the mechanisms of colour vision. In general, cichlids possessed complements of four to five cone pigments, and these complements varied across species, sexes, and individuals. Additionally, lens transmission, cone pigment expression, post-receptoral sensitivity, and retinal circuitry differed across life stages of individuals. My results suggest that the diversification of colour vision across species and across life stages of individuals contributes to sensory adaptations that enhance both the contrast of zooplanktonic prey, and the detection of optical signals from conspecifics. Therefore, both natural and sexual selection may have worked in concert to shape colour vision in fish. Since light is more complex under water than on land, fish required four to six cone classes to reconstruct the colour signals reflected from aquatic objects. This suggests that the large number of cone pigments in fish have likely evolved to enhance the reconstruction of the complex colour-signals in aquatic environments. Taken together, these findings improve our understanding of the variable nature of fish colour vision, and, more generally, help unravel the evolution of photoreceptors and colour vision.